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Highwall Mining

From RitchieWiki

Highwall mining is a method of mining that originated from auger mining. The method differs in that continuous miners, rather than augers, are used to bore an entry adjacent to the coal seam of a highwall left behind in an open pit mine after excavation has been completed. Screw conveyors positioned behind the continuous miner haul the cut coal from deep within the seam up to an outside stockpiling area where it is then transported away. Another primary difference in a highwall mining operation is that it is carried out by remote control at the surface where an operator located in a cabin uses a television camera to monitor and control the progress of the continuous miner machine.[1]

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Historically, when an open-pit or open cut mine was excavated, a highwall remained after excavation and was abandoned or covered up. This eventually provided an incentive for coal miners to devise a means of extracting coal from an existing highwall, as much labor had already gone into excavation and exposing an accessible coal seam. The primary goal for establishing a highwall mining system was therefore to increase the production life of an open-pit mine.[2]

Initial attempts to mine highwalls were met with very little success until the development of coal auger mining in the 1940s. In this respect, highwall mining has largely been viewed as an adaptation of auger mining. The problem with deploying just augers to mine a highwall was their restriction to penetrate coal seams to a limited depth. They were also unable to easily navigate over the rolls and dips of coal seams because of the rigid structuring of auger flights.[3]

Eventually, the continuous miners used in the underground mining of coal were developed and outfitted to also recover coal from surface highwalls. This spurned the development of new highwall mining machine systems such as the Terex SHM,[4], the Highwall Hog, and the Addcar System developed in 1990 by Addington, a medium-sized Kentucky-based coal mining company.[5]

When the cost of excavating an open-pit mine is no longer economically feasible, highwall mining may be deployed before the entire operation is taken underground. The economics of open-pit mining are often determined by stripping ratio. When the cost associated with removing overburden surpasses the actual value of the coal exposed in stripping or excavation, a mine may choose another method of mining the coal, such as highwall mining or taking the mining operation underground.[6]

Highwall mining as a first option presents many advantages to underground mining because it is more cost-effective and involves less lead-time.[7] The process of mining a highwall with continuous miners is proven to also be much safer since operations are carried out exclusively by remote control and require fewer personnel than underground mining. Highwall mining at the surface produces the same amount of output as mining underground might. With highwall mining, smaller blocks of coal can also be accessed and operations are facilitated around geological impediments or structures that otherwise impinge upon coal production.[8]

The process begins with coal being extracted from a reserve situated well beyond the reach of conventional surface mining methods. A standard highwall miner or mining system (HWM) involves boring usually a rectangular shaped entry or section out of an exposed highwall seam by remote control. Advanced highwall mining systems such as the Bucyrus - SHM are self-contained, hydraulically-powered tracked mobile units with electric drive. The Bucyrus - SHM highwall mining (1997) where the first HWM units that could mine parallel entries of a coal seams with a thickness ranging from 30 inches (76 cm) to 16 feet (5 m) to pre-determined depths of up to 1,000 feet (305 m).[9] A cutterhead attached to a powerhead assembly with 20-foot (6-m) pushbeams is used to penetrate and bore into the coal seam. The mined coal is then transported back to the machine by two counter-rotating screw conveyors positioned inside the pushbeams. The conveyors feature a coupling system that allows each auger to drive the next auger. The Bucyrus - SHM system is highly efficient, with the capability of producing an output of 40,000 to 120,000 tons of coal per month, depending on the height of the actual coal seam. Another benefit of the system is that it is designed to handle even wet coal together with high water-inflow and work with laser gyro guiding systems.[10]